1. Field of the Invention
The present invention relates to a method for producing iron carbide and, more particularly, to a method for producing iron carbide from iron oxide sources such as iron ore and by-products from metallurgical processes such as iron and steelmaking.
2. Background of the Invention
All major steelmaking processes require the input of iron bearing materials as process feedstocks. If the steelmaking method uses a basic oxygen furnace, the ion bearing materials are usually blast furnace hot metal and steel scrap. To augment scrap supplies in times of high demand and to dilute the impurities associated with scrap, steelmakers seek and use alternative iron sources in addition to conventional hot metal and scrap. The most well known and broadly used alternative iron source is a product known as Direct Reduced Iron (DRI) which is produced by the solid state reduction of iron ore without the formation of liquid iron. DRI is produced from ore and is much purer than scrap sources of iron units. DRI is therefore valuable to the steelmaker as a means of diluting the impurities brought into the steel melt from scrap.
Direct reduction processes for making iron are a marked departure from blast furnace methods. In the direct reduction process, reduction of iron ore oxides to iron is accomplished using a mixture of hydrogen and carbon monoxide gases at temperatures of less than 1000.degree. C. Although many direct reduction processes exist, the dominant and most successful processes are the Midrex shaft furnace method and the retort process developed by Hojalata y Lamina (the HyL Process). In both of these processes, the reducing gas mixture is produced from natural gas.
Most other DRI processes employ carbon as the reductant, usually in the form of coal. Many of these processes utilize kilns, an example being the SLRN Process or a rotary hearth furnace, as used in the INMETCO Process. For various reasons, the solid carbon reductant processes have achieved far less commercial success than the gas based processes.
The wide spread use of DRI in steelmaking has been limited by cost and availability factors. DRI production is only possible in locations with abundant sources of low cost ore and natural gas. Melting of DRI requires more time and energy than the melting of scrap. These factors limit DRI use, especially in the United states where energy costs are high and scrap is usually abundant.
In a recent development, iron carbide has been used as a source of iron for steelmaking. This development is shown in U.S. Pat. No. 4,053,301 (Reissue Pat. No. 32,247); U.S. Pat. No. 5,073,194; U.S. Pat. No. 5,118,479; and U.S. Pat. No. 5,137,566. This development should have a dramatic impact on future steelmaking practice.
In this new process, iron carbide is produced from iron ore fines by contacting the iron carbide with a mixture of reducing and carburizing gases consisting of carbon monoxide, hydrogen, and hydrocarbons. The main processing unit for this reaction is a fluid bed and the input gases are sourced from natural gas. The process operating temperature is lower than that for DRI making and the process uses the gas input more efficiently. These factors represent an economic improvement over DRI production methods. However, because the process is slow, it is capital intensive.
The use of iron carbide avoids the cost penalties of energy and time that are unavoidable when using DRI in steelmaking. Iron carbide is consumed rapidly in the steelmaking furnace and the carbon contained therein is an energy source. The energy consumed in melting iron carbide is much less than for DRI and, in fact, less than the energy required for melting scrap iron.
Because the use of iron carbide in steelmaking is attractive, there is a need for a method to economically produce iron carbide. The method should avoid the drawbacks of DRI production and should provide a cost effective method of producing iron carbide that can be broadly applied without the constraints of natural gas availability. The iron carbide should be able to be formed from a wide range of iron sources which may not be suitable for treatment in a fluid-bed because of size and other limitations.